来自相关“访问”表的MySQL COUNT（*）

Question

我一直在研究MySQL查询，并想知道是否有更好的方法来做到这一点（没有相关的子查询等）。有问题的查询是

SELECT 
   (SELECT COUNT(*) as num_visits FROM visit WHERE data_type='gallery'
    and gallery.id=visit.object_id) as num_visits, gallery.id
FROM gallery
    ORDER BY num_visits desc

表结构的相关部分是

gallery
-------
id

visit
-----
id
data_type --> enum('gallery','etc')
object_id --> ties to various tables

对此的任何帮助将不胜感激。

Answer 1

   SELECT COUNT(v.object_id) as num_visits, g.id
     FROM gallery g
LEFT JOIN visit v ON g.id=v.object_id AND v.data_type='gallery'
 GROUP BY g.id
 ORDER BY num_visits DESC

但更好的解决方案（从绩效角度来看）是预先计算并存储在gallery表（或其他一些）中的访问量